For many years rotary torque impulse units have been used to tighten threaded fasteners. A radial vane air motor is often used to power such impulse units. A typical rotary torque impulse unit includes a sealed fluid chamber filled with oil or other suitable hydraulic fluid. At one end of the fluid chamber, a spindle shaft protrudes through a pressure plate. Leakage from the fluid chamber must be minimized to maintain the desired output from the torque unit. During operation of the impulse unit oil will heat and expand in volume. Therefore, an accumulator reservoir is often provided to allow for expansion of the oil or hydraulic fluid and to provide a resupply of lost oil. Examples of such hydraulic impulse units are shown in U.S. Pat. No. 4,836,296, entitled "Fluid Pressure Impulse Nut Runner" and U.S. Pat. No. 4,789,373, entitled "Hydraulic Torque Impulse Generator. "
For an impulse unit that utilizes an accumulator reservoir to function properly, a small amount of fluid must be allowed to pass from the fluid chamber to the accumulator reservoir to compensate for expansion of the oil due to the increased temperature. Flow of the fluid into the accumulator reservoir must be severely limited, however, to prevent oil pressure peaks from being transmitted into the accumulator reservoir. To accomplish this, prior art units have utilized a nonresilient annular clearance seal between the spindle shaft and the pressure plate. This clearance surrounding the spindle shaft is necessarily small in area in order to prevent substantial oil pressure peaks from being transmitted into the accumulator reservoir. As some grinding clearances are a practical requirement of manufacturer, and as some wear of metal parts occurs during normal service, the clearance around the spindle shaft increases with use of the tool. This increased clearance eventually allows oil pressure peaks of sufficient magnitude to be transmitted to the accumulator reservoir. These pressure peaks may cause the inner and outer seals of the accumulator piston to flex, causing leakage through minute irregularities and scratches on the accumulator sealing surfaces. This flexing also may cause leakage in a pumping fashion as the pressure peaks occur in the fluid.
As a result of this wear and resulting leakage, current units require regular maintenance, approximately every 250,000 fasteners tightened or removed. This maintenance includes disassembling the tool, refilling the fluid chamber with oil, and reassembling the tool. Another problem with currently existing models is that it is difficult to purge air bubbles from the oil in the chamber during refilling of the unit.